Optical reflectometry and ellipsometry are commonly used to measure the thickness (t) and optical constants, refractive index (n) and extinction coefficient (k), of thin films deposited singly or in multi-layer stacks on substrates. Another major application is the measurement of critical dimensions (CD measurements), such as height, width, thickness, wall slope, etc, of fine geometric structures such as lines, trenches, vias, etc, in integrated circuits. For applications in semiconductor manufacturing, the requirements on precision, repeatability and system-to-system matching are becoming ever more demanding. Furthermore, the ever-decreasing geometries translate into ever-decreasing measurement areas, and thus the requirements for ever-smaller measurement spots. Examples of spectrometers and ellipsometers for thin film and CD measurements are U.S. Pat. Nos. 3,824,017, 4,293,224, 4,555,766, 5,867,276, 5,963,329 and 5,739,909, which are incorporated herein by reference. In addition, a thorough discussion of many prior art spectral ellipsometry systems can be found in the text by R. M. A. Azzam and N. M. Bashara entitled Ellipsometry and Polarized Light, North Holland Physics Publishing, 1988.
To address these application needs, current thin film and CD metrology systems used in the semiconductor industry typically employ both DUV-VIS (deep ultraviolet-visible) spectral reflectance at normal incidence together with off-axis rotating spectral ellipsometry in the range of 190-900 nm using focused optical beams. Some metrology systems, such as those manufactured by Therma-Wave, Inc. of Fremont, Calif., also include laser-based single wavelength beam profile reflectometry (BPR) and beam profile ellipsometry (BPE) technologies in addition to the more conventional spectral reflectometry and spectral ellipsometry technologies. Examples of BPR and BPE systems for thin film and CD measurements are U.S. Pat. Nos. 4,999,014, 5,042,951, 5,181,080, 5,412,473, 5,596,411, 6,678,046, 6,654,131, 6,813,034, 6,829,057 and 6,842,259, which are incorporated by reference.
Conventional laser-based BPR and BPE technologies have the advantages of better signal/noise ratios and smaller spot sizes, but at present are only single wavelength devices, and thus do not have the same capabilities as the spectral technologies. Thus these BPR and BPE methods are generally combined with conventional normal-incidence spectrometers and conventional off-axis spectral ellipsometers that employ rotating ellipsometric optical elements, such as polarizers, analyzers or compensators. In addition, conventional BPE measurements suffer from phase fluctuations arising from the environmentally sensitive birefringence of the high-NA focusing objective. Moreover, conventional BPR and BPE systems tend to be complex, and include many optical components and detector assemblies, thereby increasing system-to-system variances. Further, if BPR and BPE methods, as well as conventional spectroscopy and spectral ellipsometry methods, are used to analyze the same wafer, it is virtually impossible to focus these separate systems onto and analyze the same spot on the wafer, and this tends to result in increased noise and measurement error. Furthermore, phase fluctuations from the focusing objective, together with rotation artifacts of the compensator or analyzer, contribute to additional signal noise and measurement error in these systems.
There is a need for a spectroscopic approach that combines the advantages of spectral reflectometry, spectral ellipsometry and the laser-based BPR and BPE technologies while minimizing the disadvantages of these complex combination systems. A new spectroscopic BPR+BPE technology could perform all of the measurements of the different methods and would be a significant improvement over the current methods.